A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder. The pistons are disposed crown-to-crown in the bore for reciprocating movement in opposing directions. The cylinder has inlet and exhaust ports that are spaced longitudinally so as to be disposed near respective ends of the cylinder. The opposed pistons control the ports, opening the ports as they move to their bottom dead center (BDC) locations, and closing the ports as they move toward their top dead center (TDC) locations. One of the ports provides passage of the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports.
Each port includes one or more arrays of circumferentially-spaced openings through the sidewall of the cylinder. In some descriptions the openings themselves are called ports. However, in this description, a “port” refers to a circular area near an end of a cylinder in which a collection of port openings is formed to permit the passage of gas into or out of the cylinder. The port openings are separated by bridges (sometimes called “bars”) that support transit of the piston rings across the ports.
The pistons are equipped with one or more rings mounted to their crowns. The skirt, lands, and rings of each piston create a seal that prevents gas flow into or out of the port that the piston controls. Any tangential tension of a ring in its constrained state in the bore causes a radial force outward. Thermal deformation due to combustion heat adds to this force. This radial force causes the ring to deflect in an outward radial direction of the bore into the port openings as the ring traverses the port. When the ring must travel back into the bore (i.e., in an inward radial direction of the bore), which happens as the port closes and also as it opens fully, the ring must be guided radially inward of the bore.
If the geometry of a port opening edge at the bore surface is not well designed, the distance over, and hence the time during, which the ring is allowed to move radially inwardly of the bore can be too short. This shortened period to move radially can increase the inward acceleration of the ring, and hence raise the contact force and stress. This motion is called “ring clipping” (or “port clipping” or “port sticking”) and is undesirable. Ring clipping causes an overloaded condition in which the lubricant film acting between the bore and an outer ring surface which contacts the bore is pierced and asperities of the ring and bore surfaces begin to contact. This causes undue wear and increases friction, which leads to localized heating and high temperatures. These high temperatures can weaken the metals of the ring and cylinder. Weakened metals in the piston ring and engine cylinder can plastically deform when exposed to high contact stress during ring clipping. This plastic deformation of the ring and cylinder disrupts the geometry and roughens the surface texture, exposing more asperities. If the metals are active enough, then fusion can occur between the piston ring and cylinder sidewall. Fusion of plastically deformed parts can lead to scuffing, evidenced by torn, smeared, folded, and piled ring and/or cylinder material. Maximum contact stress is reduced by limiting the acceleration of the ring into and out of the port openings. Acceleration is reduced by spreading out the radial motion of the ring over time.